The present invention relates to a fuel cell system and a method, and more particularly to a fuel cell system in which excess hydrogen gas can be controlled and a method of controlling the fuel cell system.
A fuel cell system is a device in which the energy of chemical reaction caused by using fuel is converted directly into electric power.
Particularly, a fuel cell system includes a pair of electrodes (one is an anode and the other a cathode) which are separated by an electrolyte membrane located therebetween. In the system, hydrogen-rich gas is supplied to the anode (fuel pole) while an oxygen contained gas such as air is supplied to the cathode (oxidant pole) to generate electric power through the electrochemical reactions which occur on the surfaces of the two electrodes on their sides opposing to the electrolyte. The respective electrochemical reactions are as follows:
The anode reaction: H2xe2x86x92+2H++2exe2x88x92; and
The cathode reaction: 2H++2exe2x88x92+(1/2)O2xe2x86x92H2O.
In such a system, a reformer that allows methanol to react using water-vapor steam to generate a fuel gas containing a large amount of hydrogen can be used to generate hydrogen-rich gas as the electromotive fuel.
A compressor that intakes air from any outer source and compresses the air can be used to generate oxidant gas containing oxygen.
The compressed air from the compressor is supplied to, for example, an after-cooler where the air is cooled down and then to the cathode of the fuel cell while methanol gas is supplied from a fuel tank to the reformer where the methanol gas is formed into hydrogen-rich gas which is then supplied to the anode of the fuel cell.
Automotive applications of such a fuel cell system has been contemplated since such a system is advantageous over secondary battery for electric vehicle applications in terms of a standard mileage that a vehicle can be traveled, service conditions for a fuel infrastructure, and so on.
It has been further contemplated that both fuel cell and secondary battery are installed in an electric vehicle such that electric power source can be switched between them depending on the driving conditions, thereby enabling efficient supply of electric power.
Japanese Patent Application Laid-Open Publication No. H8-182208 discloses a configuration in which a fuel cell is employed as a back-up power supply for charging the secondary battery.
Japanese Patent Application Examined Publication No. H1-39069, and Japanese Patent Application Laid-Open Publication Nos. H6-174808 and H8-182208 disclose methods for detecting residual capacity level in a secondary battery.
The present inventor found, however, that when operation of a fuel cell system is stopped, hydrogen gas present in the pathway extending from the reformer to the fuel cell stack in the system may often remain as an excess gas. Continuous supply of such excess hydrogen gas in an unreacted form to the combustor may sometimes result in oversupply of hydrogen gas, i.e., the amount of hydrogen supplied to the combustor surpluses the disposal capacity of the combustor.
It is possible to use such excess hydrogen gas to generate electric power to be used for charging a secondary battery. In such a case, however, the same situation will also occur especially when the current secondary battery to which the electric power generated is supplied is already in a full-charged state if the charging capacity is not taken into consideration.
Alternatively, a fuel cell may be used as a back-up battery for charging a secondary battery. This case may also have the same situation when the current secondary battery to which the electric power from the fuel cell is supplied is already in a full-charged state if the charging capacity is not taken into consideration.
In view of the above investigations, the present invention has been made and provides a fuel cell system that can control the excess amount of hydrogen gas within a proper range as well as a method of controlling the fuel cell system.
A fuel cell system of the present invention is provided with: a secondary battery supplying electric power to an outer load; a fuel cell supplying electric power at least to the secondary battery; a charge allowance amount calculator calculating charge allowance amount level in the secondary battery; an excess electric power calculator calculating an excess amount of electric power that can be generated by an excess fuel gas remaining in the fuel cell system when the supply of electric power that is supplied to the outer load is stopped; and a controller controlling the charge allowance amount level in the secondary battery determined by the charge allowance amount calculator so as to be equal to or higher than the excess amount of electric power determined by the excess electric power calculator.
In other words, a fuel cell system of the present invention is provided with: a secondary battery supplying electric power to an outer load; a fuel cell supplying electric power at least to the secondary battery; means for calculating charge allowance amount level in the secondary battery; means for calculating an excess amount of electric power that can be generated by an excess fuel gas produced in the fuel cell system when the operation of the fuel cell is stopped; and means for controlling the charge allowance amount level in the secondary battery so as to be equal to or higher than the excess amount of electric power.
Besides, a met o o controlling a fuel cell system of the present invention is applied to a fuel cell system having a secondary battery supplying electric power to an outer load and a fuel cell supplying electric power at least to the secondary battery. Such a method of controlling a fuel cell system calculates charge allowance amount level in the secondary battery; calculates an excess amount of electric power that can be generated by an excess fuel gas remaining in the fuel cell system when the supply of electric power that is supplied to the outer load is stopped; and controls the charge allowance amount level in the secondary battery so as to be equal to or higher than the excess amount of electric power.
Other and further features, advantages, and benefits of the present invention will become more apparent from the following description taken in conjunction with the following drawings.